1. Field of the Invention
The present invention relates to a distance measuring device which emits a scanning laser beam; receives a light reflected by an object; and detects a distance from the object.
2. Discussion of Background
Conventionally, there has been proposed various devices for measuring a distance from an object by emitting a scanning laser beam over a predetermined angle. For example, FIG. 10 shows a conventional device disclosed in Japanese Unexamined Utility Model Publication No. Hei 3-95979 (JP-A-3-95979), wherein a schematical view of a mechanism for swinging a mirror is shown. In FIG. 10, numeral 4 designates a mirror for reflecting a laser beam provided on a revolving shaft 5. Numeral 40 designates a permanent magnet having a north pole and a south pole, and which is provided in the periphery of a movable coil 42 wound around a core 41. Numerals 43a and 43b designate conductive spiral springs which urge turning effects opposing to each other. First ends of the springs are respectively fixed to the end portions of the core 41 and the other ends of the springs are connected to a driving circuit 44. When an electric current is applied in the direction of an arrow 45 by the driving circuit 44, the mirror 4 is rotated in the right direction, and when an electric current is applied in the reverse direction, the mirror is rotated in the left direction. The device is constructed so that the laser beam can be scanned by the rotation of this mirror.
In this conventional device, the permanent magnet is fixed and the coil portion is rotated by applying electricity in order to swing the mirror. Therefore, a moving part is massive and the lead wires connected to the coil portion may possibly break because of the movement of the lead wire.
Further, there was disclosed a scanning radar device in Japanese Unexamined Patent Publication No. Hei 7-92256 (JP-A-7-92256). The device scans in both directions of emitting light and receiving light. In FIG. 11, a structure of a light receiving portion and the vicinity thereof is shown. A light 46 reflected by a scanning mirror 4 is received by a light receiving element 42 after passing through a condensing lens 11. The mirror 4 rotates around a revolving shaft 5. A mechanism for scanning comprises a rotor 51 substantially in a half cylindrical shape provided in the back side of the mirror 4 and a stator 52 arranged at a constant space from the rotor 51. The rotor 51 has a three layer structure made of a permanent magnet interposed between an upper plate of magnetic pole and a lower plate of magnetic pole. In both peripheries of the plates of magnetic pole, tooth profiles are formed so that the upper tooth profile and the lower tooth profile deviate from each other by a half pitch. Because the plates of magnetic pole are a permanent magnet, one of the plates is magnetized to be the north pole and the other is magnetized to be the south pole. The stator 52 has electromagnetic coils 52a, 52b and 52c having tooth profiles which are arranged oppositely to the tooth profiles 51a.
Next, steps of swinging the mirror 4 will be described. The coil portions 52a through 52c are in an initial position when electricity is not applied. When the coil 52a is applied with electricity and magnetized to be the south pole, the south pole and the north pole of the plates are moved from an initial position to settle in a position where a protrusion of one plate of a magnetic pole becomes the north pole, a recess of the other plate becomes the south pole, and these protrusion and recess are balanced with a recess of the coil 52a being the south pole. Then, when the coil 52a and the coil 52b are applied with electricity, protrusions of north pole in the plate of magnetic pole are attracted to protrusions of south pole in the coils, and south poles in the other plate of magnetic pole are repulsed by the protrusions of south pole in the coils, whereby the plates of magnetic pole and the coils are balanced after the plates of magnetic pole are moved by a predetermined value of .theta.. Then, when the coil 52a is not applied with electricity and the coils 52b and 52c are applied with electricity, the plates of magnetic pole are moved further by .theta. in the same manner. On the contrary, when the coil is applied with electricity to be the north pole, it is possible to rotate reversely. Accordingly, it is possible to rotate by a predetermined value of .theta. by applying electricity sequentially to the coils.
In this conventional device, it was possible to swing the mirror by only a predetermined value .theta. and the plurality of electromagnetic coils were necessary.
However, the conventional device had problems that the mirror and the mechanism for swinging were complicated; the number of parts composing the devices was large; the method of controlling swing motion was complicated; and successive swinging was impossible.